Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed to measure oxygen to determine the proportion of combusted gases in an intake passage of the engine. The sensor may also be used during non-EGR conditions to determine the oxygen content of fresh intake air. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio.
As such, when the intake oxygen sensor is used for EGR control, the EGR is measured as a function of the change in oxygen due to EGR as a diluent. To determine the change in the amount of oxygen, a reference point corresponding to an oxygen reading when no EGR is present is required. Such a reference point is called the “zero point” of the oxygen sensor. Due to the sensitivity of the oxygen sensor to pressure, as well as aging, there may be large deviations in the “zero point” at different engine operating conditions. Therefore the oxygen sensor may need to be regularly calibrated and a correction factor may need to be learned.
One example method for calibrating an intake gas oxygen sensor is depicted by Matsubara et al. in U.S. Pat. No. 6,742,379. Therein, a calibration coefficient is calculated to calibrate the output of an intake oxygen sensor and an intake passage pressure during selected engine conditions where the intake pressure is stable, that is, within a threshold. If the calibration coefficient is outside a threshold, it may be determined that the sensor is degraded.
However the inventors herein have recognized that such an approach may not learn the correction calibration coefficient if an EGR valve is leaking. Specifically, due to the location of the intake oxygen sensor downstream of the EGR valve and downstream of an outlet of a low-pressure EGR passage, in the event of EGR valve leakage, exhaust gas may leak out of the EGR passage and onto the sensor. The output of the oxygen sensor may consequently be corrupted and the EGR dilution estimated may be lower than the actual value. As a result, EGR control may be degraded.
In one example, a method for an engine comprises: learning a reference point for an intake oxygen sensor at a reference intake pressure during engine non-fueling conditions; and adjusting EGR flow to the engine based on an intake oxygen concentration estimated by the sensor relative to the learned reference point, and further based on a change in intake pressure from the reference intake pressure. In this way, an intake oxygen sensor can be calibrated without being effected by EGR valve leakage.
As an example, during selected engine non-fueling conditions, such as during a deceleration fuel shut-off condition, an adaptation of the intake oxygen sensor may be performed. During the adaptation, an output of the intake oxygen sensor may be monitored for a duration of the engine non-fueling condition. A relationship between the output of the sensor at a reference intake pressure may be learned and corrected for factors such as humidity. When the adaptation is complete, the output of the intake oxygen sensor may be used to estimate an EGR concentration, and thereby adjust an EGR flow. Specifically, the output may be adjusted with a pressure correction factor based on the current intake pressure and the reference intake pressure, and the corrected oxygen sensor output may be used to more accurately estimate the change in intake oxygen concentration with EGR dilution. By correcting for pressure changes, the pressure effect on oxygen sensor readings is compensated for. As such, even if an EGR valve is leaking, during non-fueling conditions, the charge leaked over the intake oxygen sensor is air. Therefore, by performing the adaptation during non-fueling conditions, even if an EGR valve is leaking, the output of the oxygen sensor can be relied on. In addition, based on a comparison of the zero point learned during DFSO conditions relative to a zero point learned during an idle adaptation of the sensor, EGR valve leakage can be diagnosed. For example, if the idle adaptation value differs from the DFSO adaptation value by more than a threshold amount, it may be determined that the EGR valve is leaking. Accordingly, EGR control may be modified. For example, instead of relying on the oxygen sensor output for feedback control of EGR flow, in the event of an EGR valve leakage, only feed-forward adjustments to EGR flow may be performed.
In this way, a relationship between an intake oxygen sensor and an intake pressure sensor can be learned, independent of the accuracy of either sensor, and used to adjust EGR flow. By performing the learning during non-fueling conditions, corruption of sensor output due exhaust residuals received from a leaking EGR valve can reduced. By comparing the DFSO adaptation to an idle adaptation, EGR valve diagnostics can also be performed. Overall, the accuracy of EGR estimation is increased, allowing for improved EGR control.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.